'Inside Llewyn Davis'
CBS Films 'Inside Llewyn Davis'

"It was clear to us from the beginning that we wanted to be watching whole performances, not little snippets of performances," Ethan said. "It's how you get to know the main character and how you get to know the scene."

To help him piece the album together Burnett brought Marcus Mumford of English folk rock band Mumford & Sons on board as associate producer. The pair had been talking about working on a project together. Then a few days after Mulligan, Mumford's wife, was cast in the film, Burnett got a call. 

"He said, 'Hey man this Llewyn Davis thing, I hate to bother you about this but, is there anything I can do?'" Burnett said. Mumford even offered to simply get people tea while they worked. "And I was like yeah," Burnett said. "Come help us. I'll take all the help I can get, certainly from the likes of Marcus."

Bringing Mumford on board was a reflection of just how well this type of folk, bluegrass and Americana inspired music has done since the Coens first collaboration with Burnett, the "O Brother Where Art Thou?" soundtrack. 

"Marcus will tell you, and a lot of young musicians like Marcus, will tell you that T-Bone's "O Brother" soundtrack is a big influence," Ethan said. Burnett argued that a shift towards a folk-rock revival had already started when the "O Brother" soundtrack came out, but the Coens agreed that the album added a spark.

Inside Llewyn Davis Justin Timberlake Oscar Isaac

"There's a lot of contemporary and very young artists working now who are very much influenced and carrying on the tradition of that music but in a contemporary, modern way," Joel said. Naturally, comparisons between the two soundtracks bring comparisons between the two films. Though the Depression-era south and 1961 Greenwich Village seem worlds apart, only 24 years break up the films' settings and they share a stylized look of muted colors and working-class grime. Llewyn embarks on a journey, an odyssey, much like George Clooney's character Ulysses Everett McGill in "O Brother," which in turn was influenced by Homer's "The Odyssey." Even the orange tabby cat that Llewyn reluctantly cares for is named Ulysses. 

"We’re conscious of connections between the two movies," Ethan said, though both brothers emphasize that the music of both contains more of a connection than the films.

"The movies are very different but the music of course is very, very much related," Joel said. "And this goes to a long-standing interest on our part in that kind of American music."

The "O Brother" soundtrack went platinum eight times, won the 2002 Grammy for Album of the Year, among others, and has sold over 7.8 million copies since its release. Do they have the same hopes for "Inside Llewyn Davis"? 

"We didn't have those hopes for that album either!" Burnett said, laughing. "That was a phenomenon and none of us could have predicted it."

"I think it'll be successful," Ethan said. 

The "Inside Llewyn Davis" album reached #68 on the Billboard 200 last week, and has so far sold 12,000 copies, according to data from Nielsen SoundScan. "Another Day, Another Time: Celebrating The Music Of 'Inside Llewyn Davis'" a concert of the album featuring performances from Joan Baez, Patti Smith, Jack White, as well as Mumford, Isaac and Mulligan was presented at Manhattan Town Hall and will air on Showtime on Dec 13. With the film opening this Friday and the subsequent televised concert, "Inside Llewyn Davis" fever could spread. 

"Hopefully people will discover this," Joel said, "and find it familiar and beautiful."

"Inside Llewyn Davis" opens in New York and Los Angeles Dec. 6 and expands Dec. 20. The soundtrack is out now on Nonesuch Records.